The present invention relates generally to magnetron sputtering apparatus, and more particularly to anode structures for magnetrons.
The spatial and temporal film thickness variations of precision optical coatings, such as multilayer antireflection (AR) coatings, generally must be held to within about plus or minus one percent to maintain the coatings desired optical properties. For economical production, it is desirable to apply the coatings in large area, in-line sputtering apparatus about one meter or greater in width.
DC reactive sputtering is the process most often used for large area commercial coating applications, such as the application of thermal: control coatings to architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. Such a system may be referred to as a continuous in-line system or simply a glass coater.
Inside the chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about three millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the cathode surface. This surface layer or material is known as the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. This magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the form of a closed loop is thus formed in front of the target. The magnetic field traps electrons from the discharge and causes them to travel in a spiral pattern. This creates more intense ionization and higher sputtering rates. The planar magnetron is described in U.S. Pat. No. 4,166,018.
Despite the development of the magnetically-enhanced planar magnetron, it was still not feasible to deposit high-precision, optical coatings, such as AR coatings, on a scale and cost sufficient to justify the use of such coatings on glazings for picture frames, display cases, architectural products, and similar low-cost products.
The simplest AR coating is twice as thick as a thermal control coating. Therefore, higher deposition rates are required to obtain a comparable production cost between the two types of coatings. Additionally, thermal control coatings can tolerate thickness variations of about plus or minus five percent without significant performance variations. Multilayer AR coatings, as noted, begin to show perceptible variations when thickness variations exceed plus or minus one percent. These variations are observed primarily as variations in the reflection color. However, as errors increase beyond the one percent range, the reflection value itself may increase dramatically.
AR coatings require a low refractive index material, such as silicon dioxide, as the outer film. At high deposition rates, and with accuracy and long term stability, this material is extremely difficult to deposit using DC reactive sputtering processes. Particularly, thickness variations tend to occur in the outer film, causing perceptible color performance variations.
The rotary or rotating cylindrical magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a cylindrical cathode. The cathode is rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases cooling problems and allows higher operating powers. The rotation of the cathode also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
If the cylindrical cathode is sufficiently long in relation to its diameter, or more precisely to the width of the intense plasma created by the magnetic tunnel field, a flat article passed beneath the cathode, such that its surface plane is parallel to the rotational axis of the cathode, will receive a film of equal thickness across its width. Care, however, must be taken to ensure that the magnetic field is constant along the cathode surface. Nonetheless, some loss of uniformity is inevitable at the extreme ends of the cathode where the magnet array is terminated. This is referred as the "end effect". Also, articles requiring a more or less uniform film can not be wider than the cathode length minus twice the "end effect" length.
If the walls of the coating chamber serve as an anode, the different path lengths from the cathode to the chamber walls can cause variations in the plasma potential along the cathode. Such variations can cause non-uniform film deposition to an extent greater than would be anticipated by the magnetic field "end effect." Under such conditions, it may only be possible to achieve film thickness uniformity of plus or minus one percent over a relatively small portion of the article or substrate being coated.
Furthermore, as sputtering progresses, coating material begins to deposit on the coating chamber walls and other internal components of the coater. As such, particularly when the coating material is an effective insulator like silicon dioxide, the electrical characteristics of the enclosure walls may change with time. This, in turn, may cause the sputtering plasma characteristics to change, causing, over time, variations in uniformity and film thickness. These effects can combine to shorten the useful operating period of the machine, if the film thickness changes exceed the tolerable accuracy limits for the coating being applied.
In view of the foregoing, an object of the present invention is to provide a sputtering apparatus capable of depositing thin films with a uniformity of about plus or minus one percent, and capable of maintaining such accuracy for extended periods.
Another object of the present invention is to provide a sputtering apparatus capable of producing cost effective, precision AR coatings for picture framing glass, display cases, architectural products, and lighting fixtures.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and the advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.